1. Field of the Invention
The present invention relates to a plate filter press for the mechanical separation of solids from liquids, such as oil or water, in suspensions.
2. Discussion of the Background
Filter presses are employed in many situations, such as the separation of crystallized fat suspensions, for mechanically removing the oil from paraffin and naphthalene suspensions, for mechanically drying slurries, for mechanically separating solvents from chemical or pharmaceutical suspensions and other separating processes. However, a combination of mechanical and thermal separation for complete phase separation in an apparatus which can also use temperature to clean the filter medium when the apparatus is not in use is not known. Moreover, there is no easy way to adjust the temperature of the suspension within the press during operation, i.e., to selectively heat and/or cool it. Moreover, in many cases the pressures realizable are insufficient to ensure adequate preliminary mechanical separation.
Plate filter presses employed for this purpose, which operate according to the frame or chamber filter principle, are composed of several filter plates arranged next to each other forming filter chambers in which a suspension is filtered. Increased pressure is typically generated by compressed air which is separated from the suspension by an impermeable, flexible membrane, e.g. a rubber membrane. The use of compressed air has the drawbacks that: (1) in large presses and at high pressures, considerable volumetric pressures are attained which potentially are very dangerous and, (2) compressed air is unsuitable as a heat transfer medium for temperature adjustment purposes.
A further drawback of the prior art plate filter presses is that generally they must not be operated in an empty state. This would raise the danger of destruction of the membrane.
In many cases, the membranes serve as seals for the chambers against the environment. The membranes must then absorb the entire plate closing force which often results in a much higher surface load on the membranes than the pressure within the chambers. This generally disadvantageously limits the maximum permissible operating pressure of plate filter presses to less than 20 bars.
Particularly in chamber filter presses, frequently the suspension is introduced and the filtrate is extracted through the membrane. Especially at higher pressures, this creates sealing problems and stress peaks in the membrane.
In conventional chamber filter presses, the filter chamber is surrounded by membranes on all sides. In this case, the membrane must serve as a support for a filter cloth. A nubby surface configuration on the membrane provides an opportunity for the fluid, e.g. olein, to flow off parallel to the filter surface. These membranes have the drawback that they can only withstand pressures up to a maximum of about 16 bars. At higher pressures, the nubs are compressed to such an extent that the fluid discharge channels become clogged. Moreover, the membranes have a thermal insulating effect and thus are not suitable as heat transfer members. Additionally, such surface configurations on the membrane create stress peaks which result in increased susceptibility to wear. Additional stress peaks in the membrane during operation occur as a result of typical nonuniformity of the plates' round configuration.
Another drawback of conventional chamber filter presses is that the press cakes left after fluid has been removed have irregular convex or concave shapes on both sides. Compared to a completely planar surface configuration, this invariably results in worsened release or slide-off of the filter cake once the chambers have been moved apart.
In conventional chamber filter presses, woven metal fabrics can not be employed because of their lack of flexibility. Only textile filter cloths are usable. However, compared to woven metal fabrics of the same pore size, filter cloths have the following drawbacks: (1) greater fluctuations in pore size and thus poorer crystal size separation selectivity; (2) greater flow resistance and thus longer pressing times; (3) more difficult cleaning; and (4) poorer cake release behavior.
In contrast to chamber filter presses, frame filter presses have the considerable drawback that, when the filter and membrane plates have been moved apart, the press cake gets stuck in the fixed chamber frame and does not drop out on its own.
Filter presses which can handle high pressures of more than 100 bars are also known, for example, cocoa butter presses using multi-chamber pots. Presses using multi-chamber pots have the following drawbacks: (1) they require a comparatively large amount of material; (2) they require a complicated apparatus for releasing the cake after pressing; and (3) because of the large number of nested, mutually displaceable, yet sealing, metal components, they require very high manufacturing tolerances. Thus, presses using multi-chamber pots are very heavy and expensive.
Filter presses such as the press disclosed in British Patent No. 907,485, also are used at very high pressure ranges. This press has one chamber defined in part by a filter and in part by an opposed elastically, extensible, liquid impervious membrane. First the membrane is expanded by a suspension to be filtered, then fluid on the opposing side of the membrane forces the liquid from the suspension through the filter. This press has the drawback that, at the end of the pressing process, the press cake has a closed, cylindrical shape which adheres to the filter medium and cannot be removed from the press without been broken. Removal of the cake can cause considerable difficulties when cake fragments cling to the filter medium or fragments drop down and wedge in annular gap of the cylindrical chamber. Moreover, cleaning of the membrane and filter medium requires the disassembly of the apparatus since they are otherwise inaccessible.